As an example employing a multilayer piezoelectric element, piezoelectric actuators in which piezoelectric layers and metal layers are alternately stacked one upon another have conventionally been proposed. In general, the piezoelectric actuator can be classified into the following two types of simultaneous sintering type and multilayer type in which piezoelectric porcelains consisting of a piezoelectric body and metal layers of plate-like body are alternately stacked one upon another. Among others, the simultaneous sintering type piezoelectric actuators are often used from the viewpoints of lower voltage and manufacturing cost reduction. The simultaneous sintering type piezoelectric actuators facilitate a reduction in layer thickness and have excellent miniaturization and durability.
FIG. 21(a) is a perspective view showing a conventional multilayer piezoelectric element. FIG. 21(b) is a partial perspective view showing the stacked state of piezoelectric layers and metal layers in FIG. 21(a). FIGS. 22 and 23 are partially enlarged cross sections showing the stacked structure in the conventional multilayer piezoelectric element. As shown in FIG. 21, the multilayer piezoelectric element is composed of a stacked body 103, and a pair of external electrodes 105 formed on opposed side surfaces, respectively. The stacked body 103 is configured by alternately stacking piezoelectric layers 101 and metal layers 102. Inactive layers 104 are stacked on both end surfaces of the stacked body 103 in the stacking direction, respectively. The metal layers 102 are not formed entirely over the main surfaces of the piezoelectric layers 101, thereby forming a so-called partial electrode structure. The metal layers 102 in the partial electrode structure are stacked so as to be exposed by every other layer to different side surfaces of the stacked body 103, and the metal layers 102 are connected by every other layer to the pair of external electrodes 105.
A conventional method of manufacturing the conventional multilayer piezoelectric element is as follows. That is, firstly, a metal paste is printed on a ceramic green sheet containing the raw material of the piezoelectric layers 101, in such a pattern as shown in FIG. 21(b), which forms a predetermined metal layer structure. Then, a plurality of the green sheets with the metal paste printed thereon are stacked one upon another to prepare a stacked forming body. The stacked forming body is then sintered to obtain the stacked body 103. Thereafter, the metal paste is applied to the opposed side surfaces of the stacked body 103, and then sintered to form a pair of the external electrodes 105, resulting in the multilayer piezoelectric element as shown in FIG. 21(a) (for example, refer to Patent Document No. 1).
As the metal layers 102, in general, an alloy of silver and palladium is often used. In order to simultaneously sinter the piezoelectric layers 101 and the metal layers 102, the metal composition of the metal layers 102 is often set to a 70% by mass of silver and a 30% by mass of palladium (for example, refer to Patent Document No. 2). The following is the reason that the metal layers 102 composed of the alloy of silver and palladium are used instead of the metal layers consisting only of silver.
That is, the composition of the metal layers 102, which consists only of silver and contains no palladium, causes so-called ion migration phenomenon that when a potential difference is applied to between the opposed metal layers 102, the silver ions in the metal layers 102 migrate through the element surface, from the positive electrode to the negative electrode in the opposed metal layers 102. This phenomenon tends to occur remarkably in the atmosphere of high temperature and high moisture.
On the other hand, for the purpose of forming the metal layers 102 of substantially identical metal filling rate (proportion), a metal paste whose metal composition rate and metal concentration are prepared so as to be substantially the same has conventionally been used. When this metal paste is screen-printed on the ceramic green sheet, the stacked body 103 is prepared by setting a mesh density and a resist thickness to substantially the same condition. In the metal layers 102 formed with this metal paste, voids 102′ can be formed nearly uniformly, as shown in FIG. 22.
As shown in FIG. 23, for the purpose of forming the metal layers 102 of substantially identical thickness, a metal paste whose metal composition rate and metal concentration are prepared to be substantially the same has been conventionally used. When this metal paste is screen-printed on the ceramic green sheet, the stacked body 103 is prepared by setting a mesh density and a resist thickness to substantially the same.
In the case of pressing and stacking ceramic green sheets, the metal layers 102 have a partial electrode structure. Therefore, the area where the metal layers 102 are overlapped with each other, and the area where the metal layers 102 are not overlapped with each other have different pressed states. As a result, the metal layer density may become non-uniform even in the same surface of the metal layer 102. Hence, there has been proposed the method in which the metal filling rate is equalized by forming recess portions in a ceramic sheet corresponding to the area where the metal layer 102 should be formed (for example, refer to Patent Document No. 3).
In the case of using the abovementioned multilayer piezoelectric element as a piezoelectric actuator, it can be driven by connecting and securing lead wires (not shown) by soldering to the external electrodes 105, respectively, and then applying a predetermined potential to between the external electrodes 105. The multilayer piezoelectric element used for this purpose is recently miniaturized and also required to ensure a large displacement under large pressure. Hence, the abovementioned multilayer piezoelectric element is required to be usable even under severe conditions of higher electric field (voltage) application and a long-term continuous driving.
In order to meet the abovementioned requirement, namely, the requirement of a long-term continuous driving under high voltage and high pressure, Patent Document No. 4 describes the element provided with a layer in which the thickness of the piezoelectric layer 101 is varied. That is, stress relaxation is performed utilizing the fact that the difference in thickness changes the displacement with respect to other layer.
In the simultaneous sintering type of multilayer piezoelectric element, attempts have been made to form a uniform metal layer so that a voltage can be applied uniformly to every piezoelectric body. Particularly, in order to equalize the electric conductivity of each metal layer, and equalize the surface area of the portion connected to the piezoelectric body, attempts have been made to equalize the metal composition of the metal layer. Further, in order to equalize the surface area of the portion connected to the piezoelectric body, attempts have been made to equalize the thickness of the metal layers.
In the stacked type of multilayer piezoelectric element, it has been proposed to control so that the contact resistance of the interface between the electrode and the piezoelectric body is high at the center in the stacking direction of the multilayer piezoelectric element, and is lowered toward the both ends, and so that no stress concentrates at the center in the stacking direction of the multilayer piezoelectric element (for example, refer to Patent Document No. 5).
However, unlike the normal multilayer electronic components such as capacitors, the multilayer piezoelectric element itself continuously causes a dimensional change at the time of driving. Therefore, if all of the piezoelectric bodies are closely driven with the metal layer in between, the piezoelectric element will be integrally drivingly deformed, so that the stress due to the deformation of the element is concentrated at the outer peripheral portion of the center of the element which expands at the time of compression and necks at the time of spreading. When this multilayer piezoelectric element is subjected to a long-term continuous driving under high voltage and high pressure, for the above reason, delamination might arise on the interface (the stacking interface) between the piezoelectric layer and the metal layer. Especially, stress concentrates on the interface between an active layer causing piezoelectric displacement and the inactive layer causing no piezoelectric displacement, and this interface becomes the starting point of delamination.
In some cases, resonance phenomenon that the displacement behaviors of the respective piezoelectric layers match with each other is generated which may cause beat sound, and harmonic signals of integral multiples of driving frequency are generated which may cause noise composition. When the multilayer piezoelectric element causing continuous dimensional changes are driven for a long period of time, the element temperature rises. When the energy of the temperature rise of the element exceeds heat release, there arises so-called hermorunaway phenomenon that the element temperature is raised acceleratedly. This leads to the problem that the piezoelectric body displacement is lowered as the temperature is raised, and the piezoelectric body displacement is sharply lowered by the fact that the piezoelectric layer has a higher temperature than the Curie point of the piezoelectric body. Hence, a metal layer having a small specific resistance is needed for suppressing the element temperature rise.
Further, there is the feature that the piezoelectric body displacement changes by environmental temperatures. Therefore, when the conventional multilayer piezoelectric element is used as an actuator for use in a driving element such as a fuel injector, the piezoelectric body displacement might vary by the element temperature rise. That is, due to the problem that the desired displacement varies gradually, the suppression of displacement variations during the long-term continuous operation and the improvement of durability have been demanded.
As a method of solving the above problem, the methods as described in the above Patent Document No. 4 and Patent Document No. 5 have been employed, however, it cannot be said that the improvements are sufficient under severe conditions of a long-term continuous driving at high voltage and high pressure. That is, stress may concentrate at the outer periphery of the center of the element, and the displacement may vary by the occurrence of cracks and flaking.    Patent Document No. 1: Japanese Unexamined Patent Publication No. 61-133715    Patent Document No. 2: Japanese Unexamined utility model Publication No. 01-130568    Patent Document No. 3: Japanese Unexamined Patent Publication No. 10-199750    Patent Document No. 4: Japanese Unexamined Patent Publication No. 60-86880    Patent Document No. 5: Japanese Unexamined Patent Publication No. 06-326370